Projected free fall trajectories. II. Human experiments.

We are very adept at using the purely two-dimensional information we get from our retinae to manoever and react to the three-dimensional world: witness the tennis player returning a 100 mph serve. In another article (Saxberg 1987) we have shown how gravity can in theory be used as a constraint to determine the initial conditions of a three-dimensional free fall trajectory from the two-dimensional central projection of the trajectory. We have developed a simple video game to investigate what information is important to humans trying to solve a problem of this sort: predicting where a ball will fall. We show that humans do not seem to use the trajectory information suggested by the theoretical results in (Saxberg 1985), but rely on other sources of information, such as the size of the image on the projection surface.